The present invention relates to an inflatable plug for sealing the stud holes in a nuclear reactor vessel. More specifically, the present invention relates to an inflatable plug for sealing the blind bore holes located in the reactor vessel flange of a nuclear reactor, which bore holes are normally used to fasten the reactor vessel head to the reactor vessel by means of bolts or studs passing through corresponding bores in a flange of the reactor vessel head, during refueling operations for the nuclear reactor. The present invention also relates to a method of inserting the plugs into the bores or stud holes.
The refueling of pressurized water nuclear reactors is, in general, a routine operation carried out with a high degree of reliability. For normal load requirements, refueling is provided on approximately yearly intervels, with the complete refueling operation normally taking approximately three to four weeks.
In most commercial plants, the reactor vessel is positioned within a concrete cavity in the containment arrangement, with the cavity having an upper portion above the vessel which defines the refueling chamber or refueling canal. This refueling chamber or canal is maintained dry during normal reactor operation. However, during the refueling of the reactor, the refueling chamber or canal is filled with water, with the water level being sufficiently high to provide adequate shielding so as to maintain the radiation levels within accepted limits when, after removal of the reactor vessel head or top, the fuel assemblies are removed completely from the reactor vessel. The height of such flooding water in the refueling chamber or canal is, for example 7 to 8.5 meters, and an additive, for example boric acid, is generally added to the water in order to ensure subcritical conditions during refueling.
As indicated above, in order to remove the fuel assemblies from the reactor vessel, it is initially necessary to remove the head or top of the reactor vessel. This head or top of the reactor vessel is conventionally fastened to the reactor vessel itself by means of a plurality of head bolts or studs which extend vertically down through bores in a outwardly extending annular flange on the head or top, and threadingly engage in correspondingly located blind bores in an outwardly extending substantially horizontal flange on the reactor vessel itself. In a typical pressurized water type nuclear reactor, approximately 58 such bolts are utilized, with each bolt, depending on the size of the reactor, having a diameter of 15 to 18 centimeters and a length in the order of 76 to 92 centimeters. The bolts or studs are all removed prior to flooding of the refueling chamber or canal. The head or top, however, is removed only after flooding with water.
In order to protect the threads of the stud or bolt holes or bores in the reactor vessel flange from corrosion, for example from the constituents of the water used to flood the reactor vessel chamber or canal, and from other possible damage during the refueling operation, it is customary to plug these blind bore holes prior to flooding of the refueling cavity or canal and the subsequent removal of the reactor vessel head or top. Presently, these stud or bolt holes are plugged by means of threaded plugs which are manually threaded into the individual stud holes. Such threaded plugs are not only difficult and time consuming to install, as well as to subsequently remove following the refueling operation, but moreover do not reliably provide a water tight seal for the stud holes. This difficulty in providing a reliable water tight seal is due primarily to surface irregularities in the area of the counter bore of the stud holes. As is clear, the absence of such a water tight seal can result in the undesired corrosion of the threads, resulting in considerable additional difficulty in removing the plugs, as well as in subsequently sealing the reactor vessel itself following the refueling operation.